This invention relates to group exclusion circuits and more particularly to a group exclusion circuit for key telephone systems.
There are various forms of group exclusion circuits known in the prior art. Some of the exclusion circuits are automatic and some are semi-automatic. Other than their operation, the exclusion circuits arrive at the same result. The resulting performance allows one group of telephones priority to a central office telephone line over another group of telephones. This is accomplished automatically by a high priority group telephone accessing an excluded central office line before a low priority group of telephones or semi-automatically by the high prior group telephones manually activating the group exclusion circuit.
These known group exclusion circuits are either powered by the central office line voltage or independently by a separate power supply.
There are several shortcomings of the above-mentioned exclusion circuits. These shortcomings are in the method in which the circuit is powered. Circuits operating from the central office line voltage are dependent upon central office line power. A varying line voltage will cause intermittent operation of the group exclusion circuit and in effect group exclusion will be defeated. Circuits operating from a key system power supply are independent of central office line voltage, but generally employ a multiplicity of electromechanical relays which places a large load on the power supply and reduces the efficiency of the circuit.